Without the screwheads being countersunk and plugged (generally accepted as the best method) I would be very careful sanding as the deck boards will end up with slight hollows or depressions where you sanded in between screws and that will cause more issues.

You are, of course, absolutely correct. I have no intention of sanding that much. I believe most of what I see are surface blemishes. Some of these milling lines seem to be coming off with fine sandpaper. I don't see heavy duty sanding being needed. I was wondering if others had had success using light sanding on a deck. And if I need to let it weather after sanding for the best results when I put down the UV protection.

If I may digress, your warning would have helped me in the 24 h after I bought my first house. I rented a drum sander and proceeded to work through the night sanding my oak floors. It wasn't until I had finished all the sanding, staining and poly-ing that I noticed how many valleys and hills I had created. I lived with that for 10 years before moving.

Type of mistake you only make once!!! And thing if it had been on your outside deck with rain, snow and ice.

OK, the deck is in and some test sandings indicate most of the defects can be remove by hand sanding with 120 grit followed by a general finish with 220. Not a lot of work, about 5 min per board for my medium deck. Yes, there are some perpendicular scratches that won't come out with this light sanding but otherwise it removes the milling marks and in general seems to even out the look.

My plan is to sand and then give it 6 weeks of weathering before putting on the Messner's UV protection. Would you say a simple cleaning with water and a scrub brush followed by drying would be enough preparation for a massarantuba deck a couple of days before the treatment?